The present invention relates to a thermal processing method and an apparatus therefor.
Semiconductor devices such as DRAM devices are becoming even more closely integrated, and it has become necessary to be resourceful in the structure and fabrication of such devices. For example, multi-layer films of either an SiO.sub.2 /Si.sub.3 N.sub.4 /SiO.sub.2 or an SiO.sub.2 /Si.sub.3 N.sub.4 structure are currently being investigated as capacitor insulation layers for DRAMs, in order to reduce the temperature while ensuring the insulating withstand pressure at the corners of trenches.
In order to ensure high levels of reliability for such devices, there are even greater demands for improvements in the quality of thin films because if, for example, oxygen is accidently incorporated into an Si.sub.3 N.sub.4 layer, the permittivity will drop and long-term reliability will deteriorate. Therefore, when such a multi-layer insulating film is formed, it is appropriate to use a vertical thermal processing apparatus that draws in as little as possible of the outside atmosphere into its thermal processing chamber.
When SiO.sub.2 and Si.sub.3 N.sub.4 layers are formed in the prior art, for example, the steps shown in FIG. 14 are implemented. In other words, the semiconductor wafer (hereinafter abbreviated simply to "wafer") is placed on a holder means (a wafer boat) and conveyed thereon into a reduced-pressure chemical vapor deposition (CVD) chamber at one thermal processing station, and gases such as SiH.sub.2 Cl.sub.2 and NH.sub.3 are used as processing gases to form an Si.sub.3 N.sub.4 film on the surface of the wafer while the interior of the chamber is held as a predetermined degree of vacuum. The wafer is then conveyed out of the chamber and is removed from the holder means. Next, the wafer is accommodated in a wafer carrier and is conveyed to another thermal processing station, where it is transferred to a holder means and is conveyed into an oxidation chamber, and an SiO.sub.2 film is formed on the surface of the wafer under normal pressure by exposure to oxygen or a mixed gas of, for example, oxygen and hydrogen chloride, and then the wafer is conveyed out of the chamber.
However, with the above described processing method, since the wafer with the Si.sub.3 N.sub.4 film formed thereon has to be conveyed out of the reduced-pressure CVD chamber and into a different thermal processing station, a natural oxide layer will form over the Si.sub.3 N.sub.4 layer. This is particularly likely to happen after film formation in a reduced-pressure CVD chamber. When a cap at a lower edge of the chamber is opened, it is inevitable that a certain amount of the outer atmosphere will be dragged into the chamber, and thus the surface of the Si.sub.3 N.sub.4 layer will come into contact with the atmosphere while at a high temperature. This causes a natural oxide layer of an uneven thickness to be formed, and it is extremely difficult to remove this natural oxide layer, even if the wafer is washed before it goes on to the subsequent oxidation step. Similarly, when the wafer is transferred to the holder means under the oxidation chamber, the wafer is exposed to the atmosphere while it is still at a fairly high temperature, and thus the formation of a natural oxide layer is promoted and the desired oxide layer will be formed in the oxidation chamber over the natural oxide layer. This means that there will be an oxide layer of poor quality in the multi-layer insulating film of the wafer, so that the reliability of the device, such as a DRAM, will be low.
With the prior art processing method, the wafer has to be transferred a large number of times while it is conveyed out of the reduced-pressure CVD chamber, conveyed toward the other thermal processing station while accommodated in the carrier, and them conveyed into the oxidation chamber, so that particles can easily adhere thereto. Since it seems likely that the above described multi-layer insulating film will become even thinner as DRAMs become even more densely integrated in the future, the inclusion of even a tiny amount of particles will adversely affect the characteristics of the insulating layer even further. The above problems will also occur with an Si.sub.3 N.sub.4 layer on an SiO.sub.2 layer.
Thus it is difficult to obtain an SiO.sub.2 /Si.sub.3 N.sub.4 or SiO.sub.2 /Si.sub.3 N.sub.4 /SiO.sub.2 multi-layer insulating film of a good film quality with a prior art thermal processing method, and this is one reason why the further integration of devices such as DRAMs is impeded.
Another problem with reduced-pressure CVD concerns the use of a manifold made of stainless steel that can be used for several different types of processing. This manifold is corrosion resistant, but corrosion will still be promoted if the processing temperatures increase further when highly corrosive gases are used. This means that there are limitations on the types of gas that can be used at high temperatures.
With the present invention, the inventors propose a vertical thermal processing apparatus that can perform both reduced-pressure CVD and either oxidation or diffusion in a common chamber (see the description herein of the first embodiment). It is possible that hydrochloric acid (HCl) could be mixed into the processing gases used for oxidation. The use of HCl has advantages in that it eliminates any impurities that may be introduced from the outside, and thus ensures a good-quality film, but the presence of any H.sub.2 O vapor will result in extremely strong corrosiveness, the inner peripheral surface of the stainless steel manifold will be corroded thereby, and thus contamination of the wafers by particles and heavy metals will occur. Note that forming the manifold of quartz would solve this problem of corrosion, but the manufacture of such a manifold would be difficult and expensive, and it would also be difficult to connect the external piping and intake/exhaust ports.